// Tencent is pleased to support the open source community by making RapidJSON available.
// 
// Copyright (C) 2015 THL A29 Limited, a Tencent company, and Milo Yip. All rights reserved.
//
// Licensed under the MIT License (the "License"); you may not use this file except
// in compliance with the License. You may obtain a copy of the License at
//
// http://opensource.org/licenses/MIT
//
// Unless required by applicable law or agreed to in writing, software distributed 
// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR 
// CONDITIONS OF ANY KIND, either express or implied. See the License for the 
// specific language governing permissions and limitations under the License.

#ifndef RAPIDJSON_STRTOD_
#define RAPIDJSON_STRTOD_

#include "../rapidjson.h"
#include "ieee754.h"
#include "biginteger.h"
#include "diyfp.h"
#include "pow10.h"

RAPIDJSON_NAMESPACE_BEGIN
    namespace internal {

        inline double FastPath(double significand, int exp) {
            if (exp < -308)
                return 0.0;
            else if (exp >= 0)
                return significand * internal::Pow10(exp);
            else
                return significand / internal::Pow10(-exp);
        }

        inline double StrtodNormalPrecision(double d, int p) {
            if (p < -308) {
                // Prevent expSum < -308, making Pow10(p) = 0
                d = FastPath(d, -308);
                d = FastPath(d, p + 308);
            } else
                d = FastPath(d, p);
            return d;
        }

        template<typename T>
        inline T Min3(T a, T b, T c) {
            T m = a;
            if (m > b) m = b;
            if (m > c) m = c;
            return m;
        }

        inline int CheckWithinHalfULP(double b, const BigInteger &d, int dExp) {
            const Double db(b);
            const uint64_t bInt = db.IntegerSignificand();
            const int bExp = db.IntegerExponent();
            const int hExp = bExp - 1;

            int dS_Exp2 = 0, dS_Exp5 = 0, bS_Exp2 = 0, bS_Exp5 = 0, hS_Exp2 = 0, hS_Exp5 = 0;

            // Adjust for decimal exponent
            if (dExp >= 0) {
                dS_Exp2 += dExp;
                dS_Exp5 += dExp;
            } else {
                bS_Exp2 -= dExp;
                bS_Exp5 -= dExp;
                hS_Exp2 -= dExp;
                hS_Exp5 -= dExp;
            }

            // Adjust for binary exponent
            if (bExp >= 0)
                bS_Exp2 += bExp;
            else {
                dS_Exp2 -= bExp;
                hS_Exp2 -= bExp;
            }

            // Adjust for half ulp exponent
            if (hExp >= 0)
                hS_Exp2 += hExp;
            else {
                dS_Exp2 -= hExp;
                bS_Exp2 -= hExp;
            }

            // Remove common power of two factor from all three scaled values
            int common_Exp2 = Min3(dS_Exp2, bS_Exp2, hS_Exp2);
            dS_Exp2 -= common_Exp2;
            bS_Exp2 -= common_Exp2;
            hS_Exp2 -= common_Exp2;

            BigInteger dS = d;
            dS.MultiplyPow5(dS_Exp5) <<= dS_Exp2;

            BigInteger bS(bInt);
            bS.MultiplyPow5(bS_Exp5) <<= bS_Exp2;

            BigInteger hS(1);
            hS.MultiplyPow5(hS_Exp5) <<= hS_Exp2;

            BigInteger delta(0);
            dS.Difference(bS, &delta);

            return delta.Compare(hS);
        }

        inline bool StrtodFast(double d, int p, double *result) {
            // Use fast path for string-to-double conversion if possible
            // see http://www.exploringbinary.com/fast-path-decimal-to-floating-point-conversion/
            if (p > 22 && p < 22 + 16) {
                // Fast Path Cases In Disguise
                d *= internal::Pow10(p - 22);
                p = 22;
            }

            if (p >= -22 && p <= 22 && d <= 9007199254740991.0) { // 2^53 - 1
                *result = FastPath(d, p);
                return true;
            } else
                return false;
        }

// Compute an approximation and see if it is within 1/2 ULP
        inline bool StrtodDiyFp(const char *decimals, size_t length, size_t decimalPosition, int exp, double *result) {
            uint64_t significand = 0;
            size_t i = 0;   // 2^64 - 1 = 18446744073709551615, 1844674407370955161 = 0x1999999999999999
            for (; i < length; i++) {
                if (significand > RAPIDJSON_UINT64_C2(0x19999999, 0x99999999) ||
                    (significand == RAPIDJSON_UINT64_C2(0x19999999, 0x99999999) && decimals[i] > '5'))
                    break;
                significand = significand * 10 + (decimals[i] - '0');
            }

            if (i < length && decimals[i] >= '5') // Rounding
                significand++;

            size_t remaining = length - i;
            const unsigned kUlpShift = 3;
            const unsigned kUlp = 1 << kUlpShift;
            int error = (remaining == 0) ? 0 : kUlp / 2;

            DiyFp v(significand, 0);
            v = v.Normalize();
            error <<= -v.e;

            const int dExp = (int) decimalPosition - (int) i + exp;

            int actualExp;
            DiyFp cachedPower = GetCachedPower10(dExp, &actualExp);
            if (actualExp != dExp) {
                static const DiyFp kPow10[] = {
                        DiyFp(RAPIDJSON_UINT64_C2(0xa0000000, 00000000), -60),  // 10^1
                        DiyFp(RAPIDJSON_UINT64_C2(0xc8000000, 00000000), -57),  // 10^2
                        DiyFp(RAPIDJSON_UINT64_C2(0xfa000000, 00000000), -54),  // 10^3
                        DiyFp(RAPIDJSON_UINT64_C2(0x9c400000, 00000000), -50),  // 10^4
                        DiyFp(RAPIDJSON_UINT64_C2(0xc3500000, 00000000), -47),  // 10^5
                        DiyFp(RAPIDJSON_UINT64_C2(0xf4240000, 00000000), -44),  // 10^6
                        DiyFp(RAPIDJSON_UINT64_C2(0x98968000, 00000000), -40)   // 10^7
                };
                int adjustment = dExp - actualExp - 1;
                RAPIDJSON_ASSERT(adjustment >= 0 && adjustment < 7);
                v = v * kPow10[adjustment];
                if (length + adjustment > 19) // has more digits than decimal digits in 64-bit
                    error += kUlp / 2;
            }

            v = v * cachedPower;

            error += kUlp + (error == 0 ? 0 : 1);

            const int oldExp = v.e;
            v = v.Normalize();
            error <<= oldExp - v.e;

            const unsigned effectiveSignificandSize = Double::EffectiveSignificandSize(64 + v.e);
            unsigned precisionSize = 64 - effectiveSignificandSize;
            if (precisionSize + kUlpShift >= 64) {
                unsigned scaleExp = (precisionSize + kUlpShift) - 63;
                v.f >>= scaleExp;
                v.e += scaleExp;
                error = (error >> scaleExp) + 1 + kUlp;
                precisionSize -= scaleExp;
            }

            DiyFp rounded(v.f >> precisionSize, v.e + precisionSize);
            const uint64_t precisionBits = (v.f & ((uint64_t(1) << precisionSize) - 1)) * kUlp;
            const uint64_t halfWay = (uint64_t(1) << (precisionSize - 1)) * kUlp;
            if (precisionBits >= halfWay + error) {
                rounded.f++;
                if (rounded.f & (DiyFp::kDpHiddenBit << 1)) { // rounding overflows mantissa (issue #340)
                    rounded.f >>= 1;
                    rounded.e++;
                }
            }

            *result = rounded.ToDouble();

            return halfWay - error >= precisionBits || precisionBits >= halfWay + error;
        }

        inline double
        StrtodBigInteger(double approx, const char *decimals, size_t length, size_t decimalPosition, int exp) {
            const BigInteger dInt(decimals, length);
            const int dExp = (int) decimalPosition - (int) length + exp;
            Double a(approx);
            int cmp = CheckWithinHalfULP(a.Value(), dInt, dExp);
            if (cmp < 0)
                return a.Value();  // within half ULP
            else if (cmp == 0) {
                // Round towards even
                if (a.Significand() & 1)
                    return a.NextPositiveDouble();
                else
                    return a.Value();
            } else // adjustment
                return a.NextPositiveDouble();
        }

        inline double
        StrtodFullPrecision(double d, int p, const char *decimals, size_t length, size_t decimalPosition, int exp) {
            RAPIDJSON_ASSERT(d >= 0.0);
            RAPIDJSON_ASSERT(length >= 1);

            double result;
            if (StrtodFast(d, p, &result))
                return result;

            // Trim leading zeros
            while (*decimals == '0' && length > 1) {
                length--;
                decimals++;
                decimalPosition--;
            }

            // Trim trailing zeros
            while (decimals[length - 1] == '0' && length > 1) {
                length--;
                decimalPosition--;
                exp++;
            }

            // Trim right-most digits
            const int kMaxDecimalDigit = 780;
            if ((int) length > kMaxDecimalDigit) {
                int delta = (int(length) - kMaxDecimalDigit);
                exp += delta;
                decimalPosition -= delta;
                length = kMaxDecimalDigit;
            }

            // If too small, underflow to zero
            if (int(length) + exp < -324)
                return 0.0;

            if (StrtodDiyFp(decimals, length, decimalPosition, exp, &result))
                return result;

            // Use approximation from StrtodDiyFp and make adjustment with BigInteger comparison
            return StrtodBigInteger(result, decimals, length, decimalPosition, exp);
        }

    } // namespace internal
RAPIDJSON_NAMESPACE_END

#endif // RAPIDJSON_STRTOD_
